Monoclonal antibodies prepared to spermatozoa, spermatogenic cells, or isolated antigens are being used to identify and characterize molecules specific to germ cells. The general hypothesis being tested is that germ cell-specific gene products are responsible for the unique structural and functional characteristics of spermatozoa. Sperm components being studied are: cytoskeletal proteins and surface components. The fibrous sheath is a cytoskeletal structure which underlies the plasma membrane and surrounds the outer dense fibers and axoneme in the principal piece of the mammalian sperm flagellum. A monoclonal antibody was found to recognize a protein of apparent Mr 70 K and pI 8.5 using 2D SDS-PAGE and Western blotting procedures. Antigen appearance during spermatogenesis was analyzed by ELISA on extracts of testes from mice 16 to 30 days of age. Reactivity appeared between 18 and 20 days after birth, temporally coincident with the appearance of spermatids. The hypothesis currently being tested is that the fibrous sheath protein is a germ cell-specific intermediate filament protein of the keratin family. Studies on sperm surface components involve a glycoprotein that appears on the sperm surface during epididymal maturation. A monoclonal antibody has been used to show that the antigen is secreted by cells in the corpus epididymidis and binds to the plasma membrane of the flagellum. The antibody reacts with a glycoprotein of apparent Mr 85 K present in sperm-free fluid from the corpus and cauda epididymidis. However, detergent extraction releases an apparent Mr 54 K antigen from sperm of the cauda epididymidis, suggesting that the component is modified during attachment to sperm. The antibody reacts with proteins of apparent Mr ll0 K, 86 K, and 72 K in cauda sperm extracts separated under non-denaturing PAGE conditions, suggesting that the 84 kd protein is linked by disulfide bonds to other plasma membrane-associated components.